A THEORETICAL AND WIND TUNNEL INVESTIGATION OF THE EFFECT OF CAPILLARY WATER ON THE ENTRAINMENT OF SEDIMENT BY WIND

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Abstract
A theoretical model of the effect of small amounts of water on the threshold shear velocity of sand grains has been tested in wind-tunnel studies. The model is based upon the capillary forces developed at interparticle contacts surrounded by isolated wedges of water. These forces (Fc) are inversely proportional to moisture tension (P) and directly proportional to the geometric properties of the contacts (G). Given Fc = .pi.T2G/P, the cohesion of the material decreases with increasing moisture tension and particle angularity. The incorporation of this capillary force model into Bagnold''s threshold model for dry particles gave a general relation which indicated that the threshold shear velocity of wet particles increases as the square root of the inverse of moisture tension. Laboratory measurement of the soil moisture characteristic curve of any sand therfore provides a simple, though indirect indication, of its susceptibility to entrainment by wind under low moisture conditions. Most sands appear to be exceedingly resistant to wind erosion at moisture tensions below 10 MPa or at gravimetric moisture contents above approximately 0.2%. Further work is needed to determine if this potentially practical relationship also applies to more complex agricultural soils.
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